The present invention relates generally to a high pressure sodium vapor discharge lamp using an alumina tube which is translucent or transparent and in which sodium for producing radiation, buffer gas and inert starting gas are contained.
As described in column 3, lines 58-65 of U.S. Pat. No. 3,898,504, by some of the inventors of the present invention, the following high pressure sodium vapor discharge lamp has good color rendition with color acceptability of over 1.0 in operation:
A high pressure sodium vapor discharge lamp comprising a translucent tube envelope containing therein sodium, inert starting gas, buffer gas comprising at least one of mercury and cadmium and discharging electrodes sealed in the envelope, diameter d in mm of said lamp tube and an average potential gradient E in volts/cm having the relation of: EQU E .gtoreq. 37.7-2.05d.
However, it is difficult to realize a practical high pressure sodium vapor discharge lamp having color temperature of over 2500.degree. K. Though it is theoretically possible to raise the color temperature to 2500.degree. K to 3500.degree. K simply by raising sodium vapor pressure in the discharge tube, such increase in the vapor pressure results in disadvantages of lowering efficiency and excessively high lamp voltage, thereby deleteriously affecting utility. The general color rendering index, the color temperature and the color acceptability of the lamp are defined and elucidated in C.I.E. (Commission Internationate de l'Eclairage) recommendation.